1. Field of the Invention
The present invention relates to a radiation conversion panel having a matrix of pixels for generating electric charges depending on a radiation that has passed through a subject, and a method of capturing a radiation image according to dual-energy radiography using such a radiation conversion panel.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus which apply a radiation to a subject and guide the radiation that has passed through the subject to a radiation conversion panel, which captures a radiation image from the radiation. Known forms of the radiation conversion panel include a conventional radiation film for recording a radiation image by way of exposure, and a stimulable phosphor panel for storing a radiation energy representing a radiation image in a phosphor and reproducing the radiation image as stimulated light by applying stimulating light to the phosphor. The radiation film with the recorded radiation image is supplied to a developing device to develop the radiation image, or the stimulable phosphor panel is supplied to a reading device to read the radiation image as a visible image.
In the operating room or the like, it is necessary to read a recorded radiation image immediately from a radiation conversion panel after the radiation image is captured for the purpose of quickly and appropriately treating the patient. As a radiation conversion panel which meets such a requirement, there has been developed a radiation detector having a solid-state detector for converting a radiation directly into an electric charge or converting a radiation into visible light with a scintillator and then converting the visible light into an electric charge to read a detected radiation image.
Japanese Laid-Open Patent Publication No. 2005-283262 discloses a radiation detector 200 as shown in FIGS. 10 through 12 of the accompanying drawings. The radiation detector 200 comprises a sensitive semiconductor film (hereinafter also referred to as “photoelectric conversion layer”) 201 for generating electric charges depending on the radiation that has passed through a subject, an active matrix substrate 202 for reading the electric charges generated by the sensitive semiconductor film 201, and a common electrode 203 for applying a bias voltage. The sensitive semiconductor film 201 may comprise a semiconductor film made of amorphous selenium (amorphous Se), CdZnTe, CdTe, HgI2, PbI2, or the like.
As shown in FIGS. 10 and 11, the active matrix substrate 202 has a two-dimensional matrix of individual electrodes 205 (see FIG. 10) on its surface and a charge storing and reading circuit 206 (see FIGS. 10 and 11) for storing and reading electric charges collected by the individual electrodes 205.
As shown in FIG. 11, the sensitive semiconductor film 201 and the common electrode 203 are stacked on one surfaces of the individual electrodes 205. The charge storing and reading circuit 206 comprises capacitors 206A, TFTs (Thin-Film Transistors) 206B as switching elements, and electric interconnects 206a, 206b. Each of the individual electrodes 205 is associated with one capacitor 206A and one TFT 206B.
The radiation detector 200 functions as a two-dimensional image detector which comprises a two-dimensional matrix of pixels (also referred to as “radiation detecting pixels”, “radiation detecting units”, or “radiation detecting elements”) 220, represented by an equivalent circuit shown in FIG. 12, for detecting a two-dimensional radiation image that is projected onto the sensitive semiconductor film 201 by the radiation X that has passed through a subject.
The radiation detector 200 also has a gate drive circuit 207 (see FIGS. 10 and 12) for controlling the charge storing and reading circuit 206 and a digital image signal generator 208 (see FIG. 10) for amplifying electric charges (detected charges) read by the charge storing and reading circuit 206, into a digital image signal.
The digital image signal generator 208 comprises a plurality of integrating amplifiers 211, a multiplexer 212, and an A/D converter 213.
A process of detecting electric charges with the pixels 220 will be described below. As shown in FIG. 12, a bias voltage in the range from several kV to several tens kV output from a bias supply source 222 is supplied via a bias voltage supply lead and applied from the common electrode 203 to the sensitive semiconductor film 201. The sensitive semiconductor film 201 generates electric charges depending on the radiation X that has passed through the subject. The generated electric charges are then collected by the individual electrodes 205. Specifically, the generated electric charges move to the individual electrodes 205, inducing electric charges in the individual electrodes 205. The electric charges collected by the individual electrodes 205 are then read as electric charges i from the respective individual electrodes 205 by the charge storing and reading circuit 206.
Specifically, the gate drive circuit 207 successively applies reading signals to the gates of the TFTs 206B via the electric interconnects 206a. At the same time, the multiplexer 212 successively switches to the electric interconnects 206b that are connected to the sources of the TFTs 206B to which the reading signals are applied, supplying the electric charges stored in the capacitors 206A as electric charges (current signals) i from the TFTs 206B via the electric interconnects 206b to the integrating amplifiers 211. The current signals i are amplified by the integrating amplifiers 211, and sent as radiation detection signals of the respective individual electrodes 205 from the multiplexer 212 to the A/D converter 213, which converts the radiation detection signals into a digital image signal.
Japanese Laid-Open Patent Publication No. 2002-325756 discloses a dual-energy subtraction process for applying radiations having different energy levels at an interval of about 0.2 second to a subject which has certain different structures (e.g., a soft tissue and a bone) having respective inherent X-energy absorption characteristics, obtaining two digital image signals from a flat radiation conversion panel as an X-ray detector which is exposed to the radiations that have passed through the subject, weighting the two digital image signals with respective weighting coefficients, and subtracting one of the weighted digital image signals from the other to extract an image of one of the different structures, i.e., a soft tissue image or a bone image.
Japanese Laid-Open Patent Publication No. 2003-284710 discloses a dual-energy subtraction process for applying a radiation having a low energy level to a subject which has certain different structures (e.g., a soft tissue and a bone), reading image data from only 1024×1024 pixels of all the 2048×2048 pixels of an X-ray detector which is exposed to the radiation that has passed through the subject, then applying a radiation having a high energy level to the subject, reading image data from all the 2048×2048 pixels, and using the read two image data to produce an image representing only the soft tissue and an image representing only the bone.
According to the dual-energy subtraction process (also referred to as “two-shot energy subtraction process”) disclosed in Japanese Laid-Open Patent Publication No. 2002-325756, after the first energy level is applied to the X-ray detector, the first energy-level image is read from the X-ray detector over a reading time (which is described as 130 milliseconds), and thereafter the second energy level is applied to the X-ray detector. Consequently, a relatively large motion artifact is adversely produced due to the motion of the heart of the subject between the time when the first energy-level image is captured and the time when the second energy-level image is captured.
According to the dual-energy subtraction process disclosed in Japanese Laid-Open Patent Publication No. 2003-284710, when the low-energy-level image of the subject is read in the first image capturing cycle, a 2×2 binning process is performed to read an one-pixel image from two pixels in a row and two pixels in a column, so as to shorten a radiation image acquisition time up to the second image capturing cycle for thereby preventing a motion artifact from being generated. However, the image resolution of the low-energy-level image is lowered, and there is nothing disclosed in the publication about a specific circuit arrangement for performing the 2×2 binning process.